1. Field of the Invention
The present invention relates to a non-aqueous electrolyte storage element such as non-aqueous electrolyte secondary cell, non-aqueous electrolyte capacitor and so on.
2. Description of the Related Art
In recent years, accompanied by downsizing and enhanced performance of mobile devices, a non-aqueous electrolyte secondary cell has improved properties as a non-aqueous electrolyte storage element having high energy density and become widespread. Also, attempts are underway to improve gravimetric energy density of the non-aqueous electrolyte secondary cell, aiming to expand its application to electric vehicles.
Conventionally, a lithium ion secondary cell including a positive electrode of a lithium-cobalt composite oxide and so on, a negative electrode of carbon, and a non-aqueous electrolyte obtained by dissolving a lithium salt in a non-aqueous solvent has been widely used as the non-aqueous electrolyte secondary cell.
Meanwhile, there exists a non-aqueous electrolyte secondary cell which is charged and discharged by intercalation or deintercalation of anions in a non-aqueous electrolyte to a positive electrode of a material such as conductive polymer, carbon material and so on and by intercalation or deintercalation of lithium ions in the non-aqueous electrolyte to a negative electrode of a carbon material (hereinafter, this type of cell may also be referred to as “dual carbon cell”) (see Japanese Patent Application Laid-Open (JP-A) No. 2005-251472).
In this dual carbon cell, as indicated by the following reaction formula, the cell is charged by intercalation of anions such as PF6− and so on from the non-aqueous electrolyte to the positive electrode and by intercalation of Li+ from the non-aqueous electrolyte to the negative electrode, and the cell is discharged by deintercalation of anions such as PF6− and so on from the positive electrode and deintercalation of Li+ from the negative electrode to the non-aqueous electrolyte.Positive electrode: PF6−+nCCn(PF6)+e−Negative electrode: Li++nC+e−LiCn                 →charging reaction        discharge reaction        
A discharged capacity of the dual carbon cell is determined by: an anion storage capacity of the positive electrode; an amount of possible anion release of the positive electrode; a cation storage capacity of the negative electrode; an amount of possible cation release of the negative electrode; and an amount of anions and an amount of cations in the non-aqueous electrolyte. Accordingly, in order to improve the discharged capacity in the dual carbon cell, it is necessary to increase not only a positive-electrode active material and a negative-electrode active material but also an amount of the non-aqueous electrolyte including a lithium salt (see “Journal of the Electrochemical Society, 147(3) 899-901 (2000)”).
In such a dual carbon cell, precipitation and dissolution of a lithium salt as a supporting salt may take place at any location in the cell where a non-aqueous electrolyte exists. However, precipitation of a large amount of the supporting salt on electrode surfaces causes a problem of decreased power density of the cell because the supporting salt in a solid state is an insulator.